In applicant's investigation of the particular art to which the invention is related, a generic category of power transfer switches was undertaken. Static transfer switches are so named because there are no moving parts, thus distinguishing them from electro-mechanical transfer switches. The category of solid state power transfer switches simply did not exist until recent years.
Electro-mechanical switches were discovered in the prior art that were built in a manner as to be totally irrelevant to the concept of this invention. Of all of the prior art considered, only two Patents were found which applied in any way to what may be termed a solid state static power transfer mechanism. These included the Corey U.S. Pat. No. 3,932,764 and Cheffer U.S. Pat. No. 4,484,084.
The Patent to Corey covers the method of sensing current in a preferred AC source by using its amplitude and phase to feed comparator circuits in a solid state uninterruptable power source (UPS) system, to so control the output voltage phase and amplitude of the UPS system such that the load may be transferred from the utility, preferred, AC to the UPS AC without a resulting voltage transient. A main aspect of the mechanism involved and disclosed in this application is to provide transfer between any two AC sources of any phase relationship and does not require a UPS as one of the sources. The Corey Patent on the other hand does require a UPS and its attendant phase synchronization as one source.
The patent to Cheffer covers a method of transferring a load from an AC line to the output of a DC to AC inverter using solid state devices where the output of the DC to AC inverter is held in phase synchronization with the AC line. Basically this patent does not relate to the mechanism herein disclosed because this disclosed mechanism is designed to transfer between any two AC sources of any phase relationship and therefore is asychronous.
It is well known that, in the use of high speed solid state circuitry for control and data handling operations and applications, many problems have been created. One such problem is the intolerance of equipment for aberrations in the power source, the most common of which is "power failure". Usually power failure is visualized as the absence of the power source, but in terms of equipment operation a realistic power failure is constituted by a drop in voltage sufficient to cause malfunction or damage to the equipment. Therefore, either lower voltage or no voltage are "power failures".
Early switches developed to resolve these problems included rotating equipment and mechanical transfer switches to switch the equipment from a preferred to an alternate source if the preferred source should fail, and to transfer back when the preferred source returned to an acceptable level. Many transfer switches were, and are today, electromechanical and these typically consist of a voltage sensing relay to control the preferred contactor and the alternate contactor. A voltage which drops below a predetermined value de-energizes a relay thus disconnecting the preferred contactor. As the two sources can be out of phase, a condition that would produce an interphase short circuit if the sources were connected, means were provided to insure that the preferred source was completely disconnected before the alternate source was connected. This required a mechanical interlock. For several reasons this approach has not been useable with high speed solid state circuitry which is used for control of critical processes. For example, on an instantaneous voltage failure the load is without voltage for a period of time equal to the sum of time required for voltage sensing relay operation, preferred contactor operation and alternate contactor operation. This is much too long for solid state equipment. In addition, interruption and reconnection can take place at any instant of time on the sine wave of a source voltage. This may produce arcing, intense radiated and conducted noise which may cause the solid state equipment to malfunction and which further results in transient voltage spikes which may damage the equipment. Physical contacts, for example, will bounce when they are rapidly opened or closed.
Uninterruptable power supply, UPS, systems have been developed which solve certain problems of providing power continuity to a load should the preferred source fail, but for maximum reliability an alternate source of power must also be present and accessible by the system.
Since UPS systems require alternative power from outside sources, it follows that the UPS system is in many cases an expensive redundancy. This fact suggests the use of an enhanced solid state power transfer mechanism to switch the load directly from a preferred to an alternate source without employing a UPS system. Recent developments in SCR and intelligent sensors make this goal achievable. Such an enhanced transfer system must meet certain requirements, which include means for sensing preferred source power failure to initiate the transfer, turn-off of a preferred source and turn-on of the alternate source without any adverse effects from the cause of the failure and independent of the preferred/alternate phase relationship. Critical to this situation is the restoration of voltage quickly enough to prevent any load malfunctions, and this must all be done without arcing or voltage transients and with a minimization of current surges.
It is therefore an object of the applicant's invention to provide a solid state power transfer mechanism which smoothly and effectively transfers power from a first preferred source to an alternate source rapidly and in a manner to prevent any load malfunctions.
It is a further object of the applicant's invention to provide a solid state power transfer mechanism wherein control of switching is maintained to prevent supply of power from both sources at any particular instant.
It is still a further object of the applicant's invention to provide a solid state power transfer device which insures that once the power transfer from a preferred to an alternate source has been made and the preferred source then returns to a proper operative condition, that coordinated re-transfer will automatically occur without any load malfunctions.
It is yet a further object of the applicant's invention to provide a solid state power transfer device which gives proper indication to the user thereof of the operating status of both the preferred and alternate sources and the availability of means for switching from one source to another.
It is still a further object of the applicant's invention to provide a solid state power transfer mechanism wherein a failure of the preferred source will initiate transfer to the alternate source only after the transfer mechanism insures that the preferred source current is discontinued, any difference in phase relation between the two sources thus being unable to cause an interphase short circuit; and which, upon reversal of this transfer, back to the preferred, now operative source, the same considerations are given and the preferred source is not reactivated until the alternate source ceases to deliver current to the load.
Still, a further object of applicant's invention is to provide an optional means of continuously monitoring the operational integrity of the utilized solid state switches for junction shorting and providing indication of such a shorted condition.